Aluminum metal-phosphate coating compositions for protecting metallic surfaces from oxidation and corrosion, particularly at high temperatures, are well known in the art. For example, U.S. Pat. No. 3,248,251 issued Apr. 26, 1966 to Charlotte Allen teaches a coating composition containing chromium and/or molybdenum to inhibit the reaction between an aqueous, acidic, phosphate component of the composition and a solid particulate metallic material, preferably aluminum powder.
Heretofore, phosphate-aluminum powder coating compositions for protecting metallic surfaces from oxidation and corrosion have been based on an acid-base reaction to neutralize the bonding solution and contain either hexavalent chromium or molybdenum to inhibit the oxidation of metallic aluminum. Both hexavalent chromium and molybdenum are considered toxic chemicals and are therefore environmentally disadvantageous. In particular, hexavalent chromium is rated as a carcinogen. Molybdenum is classified as a toxic heavy metal.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an oxidation and corrosion-resistant coating for metallic surfaces that does not require either chromates or molybdates to stabilize the reaction between the bonding solution and a particulate material component, e.g., powdered aluminum. Furthermore, it is desirable to have a bonding solution for such coatings that is free of both chromium and molybdenum.